The invention relates to a fastening element for two machine parts or components, such as a fit-stretch fastener and fit-stretch bolt.
Under cyclic loads or stresses, screws mainly fracture in the first stressed turn of the thread or under the head. The greatest cyclic loading which can be withstood by a material without fracturing is designated as the fatigue strength as follows: EQU .delta..sub.D =.delta..sub.M .+-..delta..sub.A
in which
.delta..sub.D =fatigue strength PA1 .delta..sub.M =mean static stress or prestress PA1 .delta..sub.A =variable stress component. PA1 due to identical stressing cross-section A.sub.s =A.sub.conc. rings a higher tensile strength is obtained than in the case of conventional stretch fasteners: EQU A.sub.stretch shank =90% A.sub.s
The maximum allowable continuous variable stress component is .delta..sub.A =60 N/mm.sup.2 for steel screws with a strength of 800 to 1200 N/mm.sup.2 and .delta..sub.A =100 N/mm.sup.2 for very high-strength screws of 1300 to 1400 N/mm.sup.2. This variable stress component is substantially independent of the prestress and the steel quality.
Knowledge of the influence of the elasticity relationship between the screw and the parts braced by it makes it possible to reduce the variable stress component acting on the screw. In the case of given cyclic stressing of a screw or bolt connection, the proportion of the cyclic load (variable stress component .delta..sub.A) acting on the thread decreases as the screw resilience becomes softer and as the connected parts of a joint become harder and stiffer.
Due to the high demands, such as for internal combustion engines of vehicles, the rotational speeds and piston speeds are increased. This leads to increased stressing of the bolts, including those bolts and/or screws forming the connection between the connecting rod base and the connecting rod cap. For example, in the case of connecting rod bolts, the latest legal requirements are for higher and more restricted tensile strength, e.g. .delta..sub.b =1100 to 1200 N/mm.sup.2 ; 1200 to 1300 N/mm.sup.2 and 1300 to 1400 N/mm.sup.2 with a prestress of approximately 90% and higher for the elastic limit .delta..sub.s.
Thus, a product has been developed for fastening together the two parts such as the connecting rod cap and base with a very high prestress in the elastic limit range of the bolt.
These requirements are satisfied by the choice of suitable material qualities and heat treatment processes, in order to obtain higher quality levels with improved elasticity characteristics and high strength. In addition, the elastic expansion must be increased by geometrical changes to the bolt.
As a result of this, a stretch bolt or fastener has been developed, which is used in the case of bolted connections exposed to permanent alternating stresses and has a thin shank, and is very elastically resilient, so that the fatigue fracture risk of the connection is reduced. This stretch fastener has a shank part with a reduced diameter, which corresponds to approximately 90% of the core diameter. The fastener is additionally provided with one or two sets of fits which cooperate with associated tolerances of the bore and having a diameter exceeding the external diameter of the fastener thread, while located in the vicinity of the center of the bolt or, optionally in the vicinity of its head. The fastener thread is located at the opposite end from the head.
Known stretch fasteners suffer from the following shortcomings. The static load carrying capacity of the stretch fastener is not the same over the entire length thereof as a result of the tapered shank. In addition, due to the diameter differences on the shank (reduced shank diameter, snug fit), the stretch fastener has discontinuities in the elongation or stretch characteristic towards the core, which prevent a uniform elastic stretching over the entire fastener length. Moreover, during cyclic stressing and due to the tranverse forces which develop and the vibration in the stretch fastener, there occurs on a closely fitting surface, corrosion at the cylindrically fitted seat in the bore resulting in a mating surface corrosion, which causes a molecular interface destruction.
This mating surface corrsosion leads to surface cracks (FIG. 6) and to fractures, which lead to serious damage to the engine. Small amplitude but high frequency waves, such as microwaves, generated by small amplitude relative movements of the fitted parts, cause local overheating and operation of the surface crystals. These hot oxide particles act in the same way as an abrasive medium and cause cracks and serious damage to the mating surfaces with ensuing fracture.
A further disadvantage is the complicated and uneconomic manufacturing method involving machining operations, such as turning and grinding during the production of fitted fasteners.